You can also shop at http://www.gqfmfg.com/cabinet-model-parts/ since they are the original manufacturer.
I tried replacing the door gasket with the stuff you can buy at the hardware store or lumberyard but it is not anywhere near as dense as the gasket you can get that is actually made for the incubator.
What is the point of the gasket? There are big ventilation holes cut into the back, why not close those some if there is air seeping around the door? Call it extra ventilation.
Personally, in this age of inexpensive solid state electronics, I'm done with wafer thermostats. It would be trivial to convert over to a solid state PID controller for less money. The one I have even has an alarm built in if the temp should vary too far. I bought 2 of these, one for the incubator and one for the hatcher I'm building next.
It's even cheaper on eBay (about $23, I think) if you aren't in a hurry (it comes by boat from China).